Upper limb amputees use prostheses for restoring the shapes and the functions of part or all of the forearms or upper arms that have been lost by accidents, diseases, or other reasons. Known prostheses include a passive prosthesis for restoring the appearance of an upper limb, an active prosthesis (active artificial hand) for restoring functions of an upper limb for enabling various movements such as grasping objects by making a part corresponding to an elbow or a hand movable with the use of a movement of the shoulder joint or the shoulder blade, and a functional prosthesis with a plurality of attachments such as hooks attached replaceable for performing specific operations. Also known are a myoelectric prosthesis, which detects changes in the small current generated when muscles contract (surface myoelectric potential) and drives a built-in motor for restoring functions such as a grasping operation. Myoelectric prosthesis is sometimes classified as one type of active prostheses.
The active artificial hand uses a looped strap commonly called harness wound around a shoulder so that the movements of the shoulder joint or the like are transmitted to a terminal device at the distal end of the artificial hand by the tension of a plurality of wires (cables) to enable the terminal device to grasp an object. The terminal device may be formed by a pair of arms, for example, which are controlled to open and close by increasing or decreasing the distance between the arms so as to hold and grip (or sandwich) an object from two directions. Also known is a terminal device formed by three arms, with the two arms (corresponding to an index finger and a middle finger) and the one arm (corresponding to a thumb) opposite thereto being controlled to open and close for performing the operation of gripping an object more stably.
The myoelectric prosthesis, on the other hand, has a sensor that detects surface myoelectric potential, and electrically controls a terminal device in accordance with the amount of change in the surface myoelectric potential detected by the sensor. With the use of these active prosthesis or myoelectric prosthesis, an upper limb amputee, being able to control a terminal device by his/her own will of action, can perform basic movements in everyday life without the aid of a care giver and improve the quality of life.
However, these active prostheses and the like had the following problems. Namely, the active prostheses are driven by using a plurality of wires, so that they have been large because of the mechanisms for driving the harnesses and wires and heavier than passive prostheses. Because of this, some users have hesitated to use the prosthesis depending on where they have been because of the rugged outlook, or found it hard to carry the prosthesis around because of its heavy weight. Also, some users have required training for a certain period of time to master the operation of the active prostheses, because the terminal devices have been mechanically driven via the tension of the plurality of wires based on subtle movements of the shoulder joint or the like on which the harness have been wound around. Sometimes, depending on the posture of the upper limb amputee, for example, when the upper limb amputee has bent down, the wires have not been successfully transmitted movements of the shoulder joint, which has imposed limitations on the control of the active prosthesis.
Myoelectric prostheses, on the other hand, are electrically controlled based on very small surface myoelectric potentials, so that they have been able to be formed compactly as compared to the wire-driven type, and also have been able to reduce their weight. However, surface myoelectric potential has been hardly generated stably and being largely dependent on individuals. Commonly known problems have been that some users simply have not been able to control myoelectric prostheses at all, or the prostheses have been prone to unwanted operations because of the unreliable control reproducibility. Another problem has been that, despite the need of teaching by a professional trainer for mastering the operation of either the active prosthesis or the myoelectric prosthesis, sometimes sufficient trainings and teachings have not been available because of the shortage in the absolute number of such trainers.